Answer:
Kisa Gotami was in deep grief at the loss her only son.
She wanted to get back her son, so she asked for medicines to her neighbours to treat her dead son.
Gautama Buddha asked her to bring some whether mustard seeds from a house where no one had ever died.
She went house to house in search of mustard seeds and asked everyone a family member had ever died or not.
She couldn't find any such house where no one had ever died.
Buddha taught her that everyone who has taken birth has to die someday